linux-kernel - [PATCH -v2] x86/entry/64: Correct the comment over SAVE_AND_SET

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Message-ID: <20200821090710.GE12181@zn.tnic>
Date:   Fri, 21 Aug 2020 11:07:10 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH -v2] x86/entry/64: Correct the comment over
 SAVE_AND_SET_GSBASE

On Thu, Aug 20, 2020 at 06:04:50PM +0000, Bae, Chang Seok wrote:
> I think somehow the “MSR write” made confusion. Our conclusion was the 
> same as Thomas' that no FENCE is needed here.

Ok, here's v2 with a corrected comment:

---
From: Borislav Petkov <bp@...e.de>

Add the proper explanation why an LFENCE is not needed in the FSGSBASE
case.

Fixes: c82965f9e530 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit")
Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/entry/entry_64.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 26fc9b42fadc..5c5d234d968d 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -840,8 +840,9 @@ SYM_CODE_START_LOCAL(paranoid_entry)
 	 * retrieve and set the current CPUs kernel GSBASE. The stored value
 	 * has to be restored in paranoid_exit unconditionally.
 	 *
-	 * The MSR write ensures that no subsequent load is based on a
-	 * mispredicted GSBASE. No extra FENCE required.
+	 * The unconditional write to GS base below ensures that no subsequent
+	 * loads based on a mispredicted GS base can happen, therefore no LFENCE
+	 * is needed here.
 	 */
 	SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
 	ret
-- 
2.21.0

Thx.

-- 
Regards/Gruss,
    Boris.

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